Also, how do these hydrodynamic interparticle forces compare against hydration/depletion solvation-type interparticle forces? Are they all of the same nature?
It seems to me that liquid bridging interparticle forces are mainly relevant for humid gas-particle systems. It also seems to me that hydration forces between particles with hydrophilic surfaces are of the same origin as interparticle lubrication forces. However, I am not entirely certain about any of these statements.
In a more general sense, for liquid-particle colloidal suspensions (e.g., solid micro- and/or nano-particles in water), what interparticle forces of hydrodynamic nature are in play?
solvation/depletion interactions are obviously of different nature than hydrodynamic interactions. For the latter (in case of net repulsive interactions) hindrance is of importance. Separation from dispersion media as well diffusion of individual particles will get slower, when particles start to 'feel each other'.
Thank you for your feedback, Dr. Titus Sobisch.
But how are hydration interactions, in particular, different from the dissipative hydrodynamic interactions explained by lubrication theory? Both seem to be due to the presence of water molecules between two approaching hydrophilic solid surfaces.
The hydration and the lubrication forces are of rather different nature. The latter acts ONLY when the particles move towards each other and originates from energy dissipation for the fluid, that is pushed out from the inter-particle gap. The range is very different: the particles start to be affected by the force at a distance of approx. 1-0.5 particle radius. One can check how this force is derived from the solution of Navier-Stokes equations in C. Crowe's book (Multiphase flows with droplets and particles).
Another story is that the conventional expression for the lubrication force (addressed to Taylor) does not entirely work for nanoparticles as the no-slip hydrodynamic condition is not applicable at the nano-scale. I refer to the works of prof. Olga I Vinogradova , who has developed expressions for lubrication forces at surfaces with partial slip.
Thank you very much for your feedback, Dr. Boris V Balakin.
1. But don't we also have a lubrication force between particles moving away from each other, with this force being attractive in that case?
2. Also, for lubrication and capillary forces, are they valid for both particles suspended in gas and liquid media?
3. Finally, for the case of nanoparticles, from a discrete viewpoint, wouldn't the no-slip BC be a function of the surface hydrophilicity? If so, doesn't that take us back to the hydration/depletion solvation-type interactions, whose nature (i.e., attractive or repulsive) is greatly influenced by surface hydrophilicity?
1. I wouldn't term this action as an "attractive". the lubrication force acts opposite motion, like a conventional friction force.
2. the lubrication force is always valid as long as the particles move relative to each other or a wall and there is a fluid between them. the capillary force needs a second fluid to form an interface: a viscous binder or a gas bubble to be presented to launch the force.
3. the partial slip is also observed at hydrophilic surfaces. in addition, it depends on roughness. not sure I get how your last statement is connected to the hydration/solvation.
My limited understanding of lubrication forces based on the papers I have been able to access is that the theoretical analyses ignore colloidal forces (as per DLVO etc) and so I would suggest caution in applying the concepts of lubrication forces to colloidal systems. I say this looking at your question as a colloid scientist and not a not someone expert in fluid mechanics so I may be incorrect.
I fully agree, Dr. John Francis Miller. That is why I am seeking a sound understanding of the physical differences between the origin of each force.
Nonetheless, there are works that "superimpose" conventional colloidal forces (vdW + EDL) with lubrication forces, such as the attached (see Sections 2.1.2.2-2.1.2.3), which raised my question.
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